Pericutaneous transluminal coronary angioplasty (PTCA) and pericutaneous transluminal angioplasty (PTA) have become common, effective procedures for treating blockages within the blood vessels or lumens of the human body. With such procedures, a catheter is navigated through the lumen to the area of blockage whereupon a balloon, navigated through the catheter to the same site, is then inflated to expand the blocked lumen and alleviate flow blockage. In order for the physician to visualize the position to which the catheter and the lumen are navigated, x-ray fluoroscopy technology has typically been employed. More specifically, a radiopaque filler material is typically embedded within the polymer matrix of the balloon and catheter, such that when the body is exposed to x-ray radiation, the radiopaque filler material, which does not transmit the x-ray radiation, becomes clearly visible upon the monitor projecting the x-ray image. Such filler materials typically include salts of barium, tungsten, and bismuth. While effective, such high-level exposures of x-ray radiation to the human body is not desirable.
Another form of imaging available to the medical community, is magnetic resonance imaging (MRI). MRI, a specialized form of nuclear magnetic resonance (NMR), is a spectroscopic technique used to obtain microscopic chemical and physical information about molecules. The radiation emitted due to the change in spin orientation is readable by MRI devices. MRI relies upon the small magnetic field or spin associated with hydrogen nuclei. The spin produces a magnetic field that is readable by MRI devices. Since the human body is comprised of approximately two-thirds hydrogen atoms, if a material is deposited or otherwise introduced into the body that alters this magnetic field or spin, MRI devices can easily identify the area where the material is introduced and thereby identify the location of the material.
MRI is a highly versatile technology that has revolutionized health care. MRI is generally directed at the protons of water and fat molecules. Protons can be thought of as mini-magnets. In the presence of a magnetic field, protons rotate or “precess” around an axis parallel to that field at what is known as the Larmor frequency. A small majority of the protons will align themselves with the main magnetic field as this represents the lowest energy level. When hit with a pulse of radiofrequency radiation that has the same frequency as the Larmor frequency, the protons can absorb this radiation and align themselves opposing the magnetic field at the higher energy level. The subsequent emission signal from the protons when decaying back to the lower energetic level can be detected and used to generate images. MRI involves such parameters as proton density, longitudinal relaxation time (T1), and transverse relaxation time (T2). MRI makes use of the behavior of protons in a magnetic field, a behavior that is influenced by a proton's environment. For example, the protons in hematomas and tumors, can be distinguished from those in normal tissue. Pulse sequences can be designed to be flow sensitive, which makes possible MRI angiography, which provides images of blood flow and the vasculature in general. One specific application of MRI angiography is to follow what brain regions are active when an individual performs particular tasks.
MRI generally requires: a magnet that generates the static magnetic field, shim coils that improve field homogeneity, a radiofrequency (RF) coil that sends the RF pulse, a receiver coil for detection of the emission signal, gradient coils that enable the localization of the emission signal, and a computer and associated software for transforming the data into an image. Proton density, T1 relaxation time, T2 relaxation time, and flow all influence emission signal intensity. The radiofrequency pulse is generally at right angles or transverse to the static magnetic field. After the pulse, the protons return to their normal magnetic state. Specifically, the translational or longitudinal magnetic field (in the direction of the static magnetic field) is restored, and the transverse magnetic field decays. T1 is the measure of longitudinal magnetization's return to its equilibrium state. T2 is the measure of transverse magnetization's return to its equilibrium state. Even though T1 and T2 are simultaneous, they can have very different effects on the final image. T1 and T2 can vary significantly from one tissue to the next, and these differences can be utilized for achieving better contrast.
Image contrast is a major concern in MRI, and there are a number of different approaches to improving contrast. One way of improving contrast is to change the pulse sequence by altering the strength, timing and multiplicity of the RF and gradient pulses. In particular, one can manipulate the time between RF pulses known as the repetition time (TR) and the time between the pulse and the subsequent echo known as the echo time (TE). Alterations in TR and TE can affect T1 and T2. When a pulse sequence is designed to highlight T1, the term T1-weighted imaging is used. T1-weighted imaging employs a short TR to accentuate the T1 effects and short TE to minimize T2 effects. T2-weighted imaging employs a long TR and long TE.
Contrast can be improved by means besides pulse sequence modification. These means include contrast agents, magnetization transfer contrast and diffusion-weighted contrast. Contrast agents generally have some magnetic property associated with them such as ferromagnetism, paramagnetism and superparamagnetism. Ferromagnetism is a characteristic of certain metals, alloys and compounds, wherein internal magnetic moments can permanently adopt a common orientation even after removal of the external magnetic field. Paramagnetism is a characteristic of certain materials, wherein internal moments can temporarily adopt a common orientation as long as supported by an external magnetic field. Superparamagnetism is a characteristic of very small ferromagnetic particles such that they behave as paramagnetic substances due to the loss of permanent magnetization because of normal thermal lattice and molecular vibrations.
The Curie point is the temperature above which a ferromagnetic material becomes paramagnetic. The principle of the Curie point and material that possesses one have been utilized in various technological areas including refrigeration and duplication of magnetic media. Paulus, et al. U.S. Pat. No. 5,429,583 describes the use of cobalt palladium seeds with a Curie point in a therapeutical range for thermal treatment of tumors. Paramagnetic contrast agents include manganese (Mn), gadolinium (Gd), and dysprosium (Dy). These and other contrast agents are sometimes contained within medical devices. Superparamagnetic contrast agents include various iron oxide compounds, as well as in general nano-sized particles containing transition metals including, but not limited to, cobalt, nickel, manganese, and chromium.
Medical devices and various other implants can actually complicate the use of MRI if those implants are magnetically susceptible. Such implants may have been unintentional, e.g. shrapnel, especially when in the eye. Pacemakers and aneurysm clips often rule out the use of MRI. However, in many medical situations, a principal aim may be to visualize an implanted device such as a catheter or stent. Ratner U.S. Pat. No. 4,989,608, Weber et al. U.S. Pat. No. 5,728,079, Young et al. U.S. Pat. No. 5,817,017 and Weber et al. U.S. Pat. No. 5,908,410 all report devices that can be visualized using MRI. There is a need for devices that give the physician, medical technicians and others improved contrast and greater flexibility using MRI to visualize medical devices.
A Peltier element transfers heat by means of electricity from one member to another, causing the former member to cool in the process. A Peltier element is designed such that when a current is applied, both positive and negative charge carriers move from the member to be cooled to the other member. Since each carrier possesses thermal energy, a Peltier element permits heat energy to move against a thermal gradient. Peltier elements have been used in such diverse areas as outer space, submarines, electronics and water coolers.
Conductive polymers are an emerging technology that have generated much attention in recent years including the awarding of the Nobel Prize for chemistry in 2000. Conductive polymers have many potential applications, which include flat screen displays, antistatic devices, catalysts, deicer panels, electrochromic windows, fuel cells and radar dishes. Conductive polymers have already found their way into some medical devices. Examples of conductive polymers include poly-p-phenylenevinylene, and polyanilines such as Panipol®. Other conducting polymers belong to the class of conjugated polymers, which include, but are not limited to the following: poly-pyrrole (PPY), poly-aniline, poly-thiophene (PtH) and paraphenylene vinylene (PPV). Further improvements can be obtained with doping of metallic elements within the polymer matrix. The conductivity of conductive polymers can be designed to be anywhere between non-conducting to metallic conducting, and can be blended with PE, PVC and many other polymers. Conductive polymers are also amendable to a wide variety of processing techniques including extrusion and injection molding.
While heating and cooling elements have been employed in medical devices, including catheters, such elements have been used principally for tissue destruction, patient warming, measuring blood flow and as a means of controlling the movement of a device. The medical community needs new methods of detecting devices using MRI, new medical devices whose visualization by MRI may be manipulated and new methods of manufacturing such medical devices. The present disclosure addresses that need by employing heating, cooling and magnetic materials to provide novel devices, methods of use and methods of construction that will deliver positive health benefits.